Technical Field
This disclosure relates to medical instruments and more particularly for reducing twist in shape sensing optical fibers in medical applications.
Description of the Related Art
Shape sensing based on fiber optics exploits the inherent backscatter in a conventional optical fiber. The principle used is distributed strain measurement in the optical fiber with characteristic Rayleigh scatter patterns, for example, in standard single-mode communications fiber. Rayleigh scatter occurs as a result of random fluctuations of the index of refraction in the fiber core. These random fluctuations can be modeled with a random variation of amplitude and phase along the length. Fiber Bragg gratings may also be employed. By using these effects in 3 or more cores running within a single length of multicore fiber, three-dimensional (3D) shape and dynamics of a surface of interest can be reconstructed.
With a 4 or more core fiber system where one core is located in the center of the cross-section, one is able to separate strain due to bending and temperature effects as long as no axial strain (tension) is applied, or if the tension is known and controllable (or can be calibrated out). Reference files store values (also called wobble) of a rate of windings of the outer cores around the central core, but when there is excessive twist, it becomes more difficult to distinguish this effect since the rate of change of the wobble increases at a non-linear rate. This often leads to inaccuracy beyond 2π torsion and instability beyond 6π torsion about the fibers longitudinal axis.
Integrating a fiber into a fixed tip Optical Shape Sensing (OSS) enabled device involves fixing a launch region of the fiber within a launch fixture, and the tip of the fiber within a distal tip of an instrument. Assuming that the launch fixture is attached rigidly to an operating table or other structure, these two fixation points will result in an accumulation of twist along the length of the fiber as the instrument is torqued during an intervention. Knowledge of this twist value is used to measure the roll degree of freedom of the instrument. However, the OSS fiber can only produce accurate shape reconstruction until approximately 2π twist about its axis in either direction, losing stability close to 6π of cumulative twist.
Twist can be introduced by the operator rotating the instrument during manipulation or, by friction between the fiber and the instrument lumen (stick-slip) as the instrument is moved. During clinical use, specifically in vascular procedures, physicians often rotate (or ‘torque’) a guidewire/instrument through multiple π turns. Such torquing leads to a build-up of twist in fixed-tip devices, which although necessary to measure the orientation of the device, causes shape sensing to lose accuracy, then stability and finally causing breakdown of shape reconstruction. This limitation severely impacts the clinical usability of fixed tip OSS devices as it imposes restrictions on the clinician. Hence, it is essential to minimize the amount of twist which is imparted on the fiber by the clinician, while ensuring that the orientation of the instrument is still known and the instrument can still be operated and torqued in the usual manner.